The present invention relates generally to an antenna using radar waves in a level gauging system and, more particularly, to an antenna comprising a plane surface from which radar waves are transmitted towards a surface of a fluid kept in a container and to which reflected radar waves are received from the fluid surface in order to gauge the fluid level in the container.
In level gauging systems using radar waves for gauging the fluid level in a container, such as a tank on a ship or the like, planar antennas are commonly used today since they are fairly cheap to manufacture. These antennas are usually mounted in an opening in the lid of the container and comprise a plane surface facing the surface of the fluid and having means for transmitting/receiving radar waves in order to gauge the fluid level. The plane surface of the antenna forms a horizontal cold wall which causes e.g. water vapour to condensate on the plane surface, especially if the container is a heated tank. Condensed fluid on the antenna surface leads to disturbed propagation of the radar waves and/or absorbed radar waves. Therefore, planar antennas are very sensitive for environments which are strongly condensing.
A radar wave type liquid level gauge is known from JP 10-197617, in which a planar antenna is provided with a conically formed protective board (radome) on which paraffin is condensed through evaporation of stored oil. The board is disposed such that the top of the cone faces the antenna at the shortest distant, i.e. the top of the cone faces the antenna. Due to the conical form of the protective board, the condensate is caused to flow down to the peripheral part of the board. The protective board is a separate unit placed at a certain distance from the antenna and does not form a part of the antenna.
The solution described above deals with the problem of leading condensate away from the antenna. There is, however, a need for a solution which, besides leading the condensate away from the antenna, minimizes the tendency of e.g. water vapour to condensate on the plane surface of the antenna.
Antennas provided with a radome placed at a distance from the antenna leaving a space filled with air between the antenna surface and the radome, may have a problem with the appearance of condensed fluid in that space if, there by any chance occurs a leakage between either the radome and the container or the antenna and the surroundings. For example DE 3 201 104 shows an antenna provided with a radome, where the antenna is upwards directed having an opening between the antenna and the radome at the edge of the radome in order to lead condensed fluid out from the space between the antenna and the radome. There is, however, a need for a solution which is applicable in a level gauging system, where the antenna is directed downwards towards the fluid surface.
The present invention is directed to overcoming, or at least reducing the effects of one or more of the problems set forth above.
In one aspect of the present invention an antenna is provided, which uses radar waves for measuring the fluid level in a container. The antenna is provided with a radome arranged and formed such that to lead condensate away.
In another aspect of the invention an antenna is provided, which uses radar waves for measuring the fluid level in a container. The antenna is provided with a protecting material having thermally insulating characteristics.
In still another aspect of the invention an antenna is provided, which uses radar waves for measuring the fluid level in a container. The antenna is provided with a radome forming a space between the antenna and the radome. The antenna is further provided with a filling material provided in said space between the antenna and the radome and being tightly fit between the antenna and the radome such, that it prevents condensed fluid to enter the space in case of a leakage between either the radome and the container or the antenna and the surroundings.
In yet another aspect of the invention an antenna is provided, which uses radar waves for measuring the fluid level in a container. The antenna is provided with a radome forming a space between the antenna and the radome, in which space a filling material is provided. The radiation of radar waves from the antenna is optimised in consideration of the radome and the filling material, why the thickness of the radome varies along the length of the radome.